Among programmable, erasable, and readable non-volatile memory devices, a NOR-type flash memory device is known for its high speed programming and reading operation.
FIG. 1 illustrates a cross-sectional view of a typical flash memory cell structure.
A flash memory cell comprises a source 3 and a drain 4, a floating gate 6, and a control gate 8. The N.sup.+ source 3 and drain 4 are formed on a P-type semiconductor substrate 2 with a channel region interposed therebetween. The floating gate 6 is formed on the channel region with a thin insulating layer of 100 .ANG. or less interposed therebetween. The control gate 8 is insulated from the floating gate 6 with an insulating layer 9 interposed therebetween. In order to apply voltages for programming, erasing, and reading, power terminals Vb, Vs, Vd, and Vg are respectively connected to the substrate 2, the source 3, the drain 4, and the control gate 8.
Conventionally, in order to program flash memory cells, hot electrons are injected into the floating gate 6 in the channel region adjacent to the drain region 4. The electron injection may be performed by grounding the source region 3 to the P-type semiconductor substrate 2, applying high voltage (10 V) to a control gate electrode Vg, and applying proper amount of voltage (5-6 V) to the drain region 4 so as to generate the hot electrons therein. If the flash memory cells are programmed by the foregoing voltage application, negative charge is stored to the floating gate 6.The negative charge stored to the floating gate 6 raises the threshold voltage of the programmed flash memory cell.
The voltage application for the reading operation is conventionally performed by applying positive voltage (+1 V) to the drain region 4 of the flash memory cell, a predetermined voltage (about 4.5 V) to the control gate electrode Vg, and 0 V to the source region 3.During the reading operation, the programmed flash memory cell that the threshold voltage is raised through the hot electron injection method may prevent current flow from the drain region 4 to the source region 3. The programmed flash memory cells are called `off cells` and their threshold voltages thereof are distributed in a voltage range of 6-7 V.
For an erasing operation of the flash memory cells, F-N tunneling (Fowler-Nordheim tunneling) is performed from the semiconductor substrate 2 (that is, bulk area) to the control gate 8, so that the flash memory cells can be erased. The F-N tunneling may be performed by applying negative high voltage (-10 V) to the control gate 8 and applying proper positive voltage (5 V) between the bulk area and the control gate 8. In order to maximize erasing, the drain region 4 maintains a high impedance state (for example, floating state). Applying the voltages to the power terminals Vg, Vd, Vs, and Vb for the erasing operation, creates a strong electric field between the control gate 8 and the bulk area. Consequently, the F-N tunneling is performed to emit the negative charge in the floating gate of the programmed cell to the source region 3.
The F-N tunneling is generally performed by applying 6-7 MV/cm field across an insulating layer, e.g., a thin insulating layer of 100 .ANG. or less disposed between the bulk area and the floating gate 6. If the negative charge is emitted from the floating gate 6 to the bulk area by the F-N tunneling, the threshold voltage of the memory cell is lowered during a series of reading operations. In a conventional flash memory structure, each bulk area is connected to a plurality of cells so as to highly integrate the memory cells. If such an erasing operation is performed, the plurality of cells are simultaneously erased. Erasure unit is decided according to an area that each of the bulk area is divided. With constant voltage being applied to the control gate 8 of a cell whose threshold voltage is lowered by the erasing operation, a current path is created from the drain region 4 to the source region. Here, the cell is called `on cell`-such cells have threshold voltages distributed in a voltage range of 1-3 V.
If a plurality of the memory cells are, however, erased by the F-N tunneling lowering the threshold voltage of the memory cell, the threshold voltage of some memory cells become 0 V or less. This is resulted from uniformity to the threshold voltage of the cells. The cells whose threshold voltage is 0 V or less are called an over-erased cell. The over-erased cell may have 0 V or more through the erase repair operation. The erase repair operation is conventionally performed by grounding a source region of the over-erased memory cell and a P-type substrate region, applying lower amount of voltage (+3 V) to a control gate electrode than during program operation (+10 V), and applying proper amount of voltage (5-6 V) to a drain region. The negative charge is stored in the floating gate of the over-erased cell less than during the programming operation, so that the threshold voltage of the cell is raised to be 0 V and more.
A [TABLE 1] illustrates voltage level applied to the power terminal Vg, Vd, Vs, and Vb during the programming, erasing, and reading operation to the flash memory.
TABLE 1 ______________________________________ Operation mode Vg Vd Vs Vb ______________________________________ Program +10 V +5-+6 V 0 V 0 V Erasure -10 V Floating Floating +6 V Erase repair +3 V +5-+6 V 0 V 0 V Reading +4.5 V +1 V 0 V 0 V ______________________________________
FIG. 2 illustrates voltage distribution of a memory cell and FIG. 3 illustrates a cross-sectional view of a memory cell array.
Since a NOR-type flash memory cell should apply positive voltage instead of 0 V to a bulk area during an erasing operation, it cannot share the bulk area with other transistors. It is, therefore, required to form a second well 30 (pocket well) in a first well 20 in the substrate 10. A P.sup.+ tapping 33 for applying voltage to the second well 30 is also required as shown in FIG. 3. Consequently, the P.sup.+ tapping undesirably increases the layout area.
In addition to this problem, if the resistance is high between the P.sup.+ tapping 33 and the cell, not only hot carriers but also hot holes are generated such that electric current flows into second well 30 during the programming operation. The hot carrier and the hot hole increase the electric field of a bulk area and turn on a parasitic bipolar transistor and then the electric field of a drain region decreases. As a result, the programming operation may not be successful because of such "snapback"phenomenon.